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Paper is the main substrate for printing, and the quality of the paper directly affects the quality of the printed products. Therefore, for printing enterprises, ensuring the quality of the purchased paper is a prerequisite for printing high-quality products. Strengthening and emphasizing the inspection and storage of paper quality is of great significance to printing enterprises. This article discusses several viewpoints on the inspection and storage of flat paper.
1. Inspection of flat paper
1) Inspection of the outer packaging of the paper products
At present, the main forms of flat paper packaging are wood plywood packaging and wooden cases packaging. For qualified flat paper, the outer packaging usually consists of two or more layers of wrapping paper, along with wooden boards, and is tightly bound with iron wire. The main types of wrapping paper used include wax paper, asphalt paper, kraft paper, and snake skin paper, etc. On the wood plywood or wooden cases, words such as "moisture-proof" and "do not throw" are noted, and some also indicate the direction of the paper's fibers. On the wood plywood or wooden cases, the product's certificate of conformity should also be pasted, along with the product name, specification, weight per piece, number of pieces per order, paper grade, standard for execution, production date, and manufacturer's name, etc. The outer packaging should be intact without any deformation or damage. If the outer packaging is severely damaged and the paper is exposed outside the packaging, it should be rejected without hesitation, or other measures should be taken to avoid unnecessary losses. The acceptance of the outer packaging of the paper is a basic acceptance, and this critical point must be strictly controlled.
2) Visual inspection after removing the outer packaging
After removing the outer packaging of the paper, first check if there is a quality inspection sheet, and then inspect the texture and color of the paper.
The normal paper appearance has the basic characteristic of good flatness, meaning that the paper is flat both on the surface and at the edges. When it comes to a specific piece of paper, it is relatively difficult to determine whether its appearance is normal, unless the paper has undergone severe deformation. The usual approach is to unwrap the paper package and stack it 1 meter high to see if the appearance is normal. When the appearance is abnormal, it takes on various shapes, such as wavy and disc-shaped. Other shapes include mountain-shaped, valley-shaped, corner-upward-shaped, bag-shaped, inclined-shaped, trough-shaped, curled-shaped, saddle-shaped, etc. The causes of these abnormal paper appearances are diverse. In summary, there are mainly the following aspects: ① Uneven moisture content throughout the paper sheet, resulting in non-uniform local contraction; ② Uneven thickness formed during papermaking; ③ Inconsistent expansion of the front and back sides of the paper, especially more obvious for single-sided coated paper.
Furthermore, by observing the sides of the stack of papers, one can also determine whether the color of the papers is uniform. In any case, the papers supplied in the same batch must have consistent whiteness, uniform color tone, and small color differences. Otherwise, when the printed materials are bound into a book, there will be obvious color layering in the top and bottom margins, the binding edges, and other areas.
3) Inspection of single sheets of paper
Randomly select a piece of paper from the stack and check whether it has any surface defects and the quality of its uniformity. Touch the surface of the paper with your hand or look at it flatly to check the degree of smoothness and cleanliness of the paper surface. Generally speaking, the surface of the paper should be smooth without any marks or bumps, and there should be no spots of varying lightness and darkness, uneven reflection, or dust that is inconsistent with the color of the paper. Hold the paper up to the light and observe it to check its uniformity. When observing under the light, the paper should not have varying degrees of light transmission or transparency. If there are large areas of uneven cloud-like spots on the paper, it indicates poor uniformity of the paper, which will seriously affect the quality of the printed product.
4) Inspection of consecutive 4 to 5 sheets of paper
Flat paper is sometimes made by cutting and stacking multiple rolls of paper simultaneously. Due to the slight differences in paper-making processes of different rolls, there are differences in the properties of adjacent sheets within the same paper stack. If an unqualified roll of paper is mixed in, a cyclic abnormal phenomenon will occur, and printing will have printing failures at certain intervals during the printing process. Therefore, it is necessary to continuously extract several sheets from the paper stack for comparison and inspection to check if there are such differences between adjacent sheets. Although this situation is relatively rare, sometimes this observation and analysis are particularly important, and we must pay attention to the accumulation of this kind of experience.
5) Inspection of Paper Size
The measurement of paper size should be conducted in accordance with national standards using a steel tape measure with a length of 2 meters and an accuracy of 1 mm. During the measurement, 3 samples should be randomly selected from the same package. The measurement result should be expressed as the average of all measurements, accurate to 1 mm. The allowable error for the length of the paper size is ±3 mm. If the paper is skewed, it will result in insufficient or excessive paper size, causing the angle between the long and short sides of the paper not to be a right angle. Therefore, in addition to measuring the length size of the paper, the skewness of the paper should also be measured. The allowable error for the skewness of the paper is 3 to 5 mm.
6) Check whether the paper weight is consistent with the specification provided by the manufacturer.
Use a paper cutter to cut 5 paper samples measuring 200×250mm. Place them on a highly sensitive balance for weighing. Then, divide the measured mass by the area of the paper to obtain the paper's basis weight. The measured result should be very close to the basis weight indicated on the product certificate. The basis weight can fluctuate within the range permitted by national standards, but it must not exceed this limit.
7) Inspection of Paper Size Stability
The stability of paper size can be expressed by the paper's elongation rate. When inspecting the paper, the elongation rate of the paper in water can be used as a measurement. Take a horizontal paper strip with a size of 2×20cm and place it in water for 1 to 2 hours to allow it to fully expand until the size remains unchanged. Then, remove it, lay it flat on a glass plate, and use a transparent ruler to measure the size. Calculate the elongation rate and compare it with Table 1.
If the paper is in a poor condition after being unpacked, and if its elongation in water is too high, it is best to reject such paper to avoid potential problems later.
8) Inspection of the surface strength of the paper
The surface strength of paper is a very important property of paper. Each type of printing paper has certain requirements for it. Therefore, the state has made clear regulations on the surface strength of each type of printing paper. If the surface strength of the paper fails to meet the requirements of printing, then during the printing process, the paper will experience shedding of fibers, powdering, and even layering. Therefore, the inspection of the surface strength of the paper is also an important part of paper acceptance, but often many printing enterprises ignore it, resulting in various printing faults during the printing process.
When inspecting paper, some simple methods can be used to determine the strength of the paper's surface. For example, the Daniels Wax Stick method can be employed. Although this method has some drawbacks, it remains a relatively simple approach for measuring the ability of a specific type of paper to resist shedding of fibers. During the measurement process, the tip of a certain grade of wax stick is melted and pressed onto the surface of the paper being tested. After 15 minutes, it is quickly removed. Then, check the paper fibers or dust that have been peeled off on the end. Another simple method is to use a slightly moist finger (usually the thumb) to press firmly onto the paper surface, gently rub it, and then quickly separate it. Observe whether there are short fibers or powdery substances adhering to the surface of the finger.
In conclusion, without any detection instruments, the quality of the flat paper can be roughly determined by the above methods. Of course, if one wants to accurately measure various quality indicators of the paper, specialized instruments and equipment are necessary, and the measurement should be conducted in a certain experimental environment. These details will not be elaborated here.
2. Storage and Precautions for Flat Sheets
When storing flat sheets of paper, the following aspects should be mainly paid attention to:
The warehouse for storing paper should be at a certain height above the ground. The warehouse should be clean, dry and have good ventilation. There should not be too many windows, which should be placed at a high position and be small in size. Curtains should be provided to prevent direct sunlight from shining on the paper. The temperature inside the warehouse should be 18-22℃, and the relative humidity should be 60-65%.
The warehouse must be dedicated. No other chemical substances or items with high moisture or oil content should be stored inside the warehouse.
When the paper is placed on the ground, wooden blocks should be placed underneath it, with the height being approximately 35 cm. At the same time, the paper should be kept about 40 cm away from the wall. During snowy or rainy weather or after rain, the warehouse should not be ventilated to prevent the paper from getting damp.
During the rainy season, it is necessary to prevent paper from getting moldy. When there is a small amount of mold on the paper, a 3% formaldehyde aqueous solution can be gently applied. If the mold problem is more severe, sulfur fluoride (SO2F2) can be used for fumigation treatment.
Plastic sheets should be placed flat. Do not stack them vertically. When stacking three layers of plastic sheets flat, when stacking further, they should be arranged in a conical shape to prevent collapse.
Smoking is strictly prohibited in the warehouse and necessary fire-fighting equipment should be provided. There should be no exposed wires on the walls and no electrical appliances with open flames are allowed to be used.
A detailed ledger should be established, indicating the production date and entry date of the paper. The paper should be issued in accordance with the principle that the newer paper is released first to prevent deterioration due to prolonged storage.
It is necessary to regularly check the condition of the paper, especially during the spring and summer seasons. Store the medicine properly to prevent insect damage and rodent infestation.
When storing the paper, do not open the package. If the packaging is damaged, try to repair it to prevent the paper from being directly exposed to the air, which could cause the paper to frequently exchange moisture with the outside environment and lead to unexpected deformation of the paper.
The temperature and humidity in the printing environment should not differ greatly from those in the storage warehouse for paper. This is to prevent the paper from undergoing significant deformation when it is unpacked and placed in the printing environment.
Certain passages should be left between different types of paper stacks to facilitate the handling of the papers.
